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Bridge rectifiers and zener diodes are commonly used in electronic products such as computers and stereos. Bridge rectifiers are composed of a four-diode array.

These diodes convert alternating current into direct current. Bridge rectifiers have four leads, and are found in power supplies. Zener diodes are used to provide

regulated voltage for power supplies. A failure of either a bridge rectifier or zener diode prevents electronic devices from working properly.

Bridge Rectifier

• Unplug the device that contains the bridge rectifier. This ensures the bridge rectifier is without power.
• Set the digital multimeter, or DMM, to the "diode test" setting per the device's instructions.
• Test the bridge rectifier diodes. Place the DMM's two leads on two adjacent rectifier pins. The DMM will read either "OL," indicating an open circuit; or a voltage of 0.7 volts DC or less. Switch the DMM leads. An operational bridge rectifier diode shows a reading opposite of the previous reading. Perform this test on each adjacent pair of bridge rectifier pins. The bridge rectifier is faulty if the readings are the same for any of the individual diodes.

Zener Diode

• Unplug the device containing the zener diode. This ensures the zener diode has no power.
• Set the DMM to the "diode test" setting.
• Test the zener diode. Place the two DMM leads on the two zener diode leads. The DMM will read either "OL" or 0.7 volts DC. Switch the DMM leads. A working zener diode exhibits a second reading opposite of the first reading.

Since the voltage dropped across a Zener Diode is a known and fixed value, Zener diodes are typically used to regulate the voltage in electric circuits. Using a resistor to ensure that the current passing through the Zener diode is at least 5mA (0.005 Amps), the circuit designer knows that the voltage drop across the diode is exactly equal to the Zener voltage of the diode.

NEW Click here to visit Zener-Diode.co.uk for a site dedicated to zener diodes, their characteristics, usage, and other information.

Zener Diode Voltage Regulator Circuit

A zener diode can be used to make a simple voltage regulation circuit as pictured above. The output voltage is fixed at the zener voltage of the zener diode used and so can be used to power devices requiring a fixed voltage. Click here to find out more about the Zener Diode Voltage Regulator and how you go about selecting the resistor and zener diode.
NEW The above mentioned article on zener diode voltage regulators has now been updated and a handy calculator has been added to help you select the correct resistor and zener diode values and power ratings.

Zener Diodes in Series With Loads

If you have a regulated fixed voltage - say 12 Volts from a desktop PC power supply, and you want to power something requiring a lower voltage, it is possible to simply place a zener diode in series with the load device. You would choose a diode with a zener voltage equal to the supply voltage minus the voltage drop across the load.

For example, if you have a 1 Watt 6 Volt lightbulb to power from a 12 Watt regulated power supply, a 6.2V zener diode could be placed in series with the bulb giving the bulb 5.8V, or you could overpower the bulb a little using a 5.6V zener diode and dropping 6.4V across the bulb. Heat would be generated in the zener diode so it is essential to calculate the power lost in it so a suitably rated diode could be chosen.

Using a 5.6V zener diode, and knowing the bulb draws a current of around one-sixth of an Amp, we can calculate the power loss in the diode (with Ohm's Law) to be just under 1 Watt. Therefore a standard 1.3 Watt zener diode should be up to the job.

Circuits with Zener Diodes

Zener diodes are key components in the circuits presented in the following articles: Make a Simple Battery Status Monitor, LM741 Voltage Indicator, and High Capacity Alternative to 9V Battery.

Rectifiers find a use in detection of amplitude modulated radio signals. The signal may be amplified before detection, but if un-amplified, a very low voltage drop diode must be used. When using a rectifier for demodulation the capacitor and load resistance must be carefully matched. Too low a capacitance will result in the high frequency carrier passing to the output and too high will result in the capacitor just charging and staying charged.

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